Long-term temperature, oxygen and water clarity trends in Swiss lakes

Abstract. Water temperature, electrical conductivity, dissolved oxygen concentration, and water clarity are key variables routinely measured in Swiss lakes by lake monitoring programs run by cantonal environmental offices and international lake commissions. In most lakes, data is collected at bi-weekly to monthly intervals; however, the data processing pipelines remain largely manual, non-automated, and decentralized, making data access and use difficult. The Swiss Federal Office for the Environment (FOEN) consolidates and stores this information at the national level, but this process is not always done on a regular basis and the data are not directly accessible to the public. This limited accessibility restricts their use in scientific research, particularly for comparative studies across different lakes. Here, we present a harmonised dataset of temperature, electrical conductivity, dissolved oxygen and Secchi depth (a widely used proxy of water clarity) collected by cantonal offices, and research institutes in 21 large Swiss lakes and lake basins from the beginning of consistent records (between 1938 and 2001 depending on the lake) to the end of 2023. In addition, we provide consistently calculated variables including lake heat content, Schmidt stability, thermocline depth and hypolimnetic oxygen. We used the measured and calculated variables to identify long term trends in large Swiss lakes. Specifically, we investigated whether the effects of climate change and re-oligotrophication led to a coherent pattern across Switzerland or whether the responses varied from lake to lake. We found a clear warming trend in all lakes with consistently increasing mean annual surface (since ~1980) and bottom temperature (since ~2010), heat content, and stronger autumn stratification, which are consistent with expected effects of climate change. Similarly, there is a general trend towards greater water clarity, consistent with re-oligotrophication. Thermocline depth and lake oxygen concentrations showed less clear patterns. For the former, observations indicate modest shifts toward shallower thermoclines in deeper lakes and towards deeper thermoclines in shallower lakes. In contrast, deepwater oxygen concentrations show no consistent trends over recent decades, potentially reflecting opposing influences of climate change and re-oligotrophication. By publishing this data, we aim to advocate for open data policies at national and international levels, facilitating its reuse in further scientific research, and contributing to evidence-based lake management and decision-making.